Having both an ether and alcohol functional group in the same molecule, glycol ethers are one of the most versatile classes of organic solvents. These molecules combine the best solvency features of alcohols and ethers, which allows for good miscibility and solvency in a wide range of organic chemicals and oils, as well as solubility in water. Glycol ethers also have higher boiling points. For these reasons, glycol ethers are prominent in the (i) surface coating industry as active solvents for resins, (ii) brake fluid industry as solvents, (iii) petroleum industry as anti-icers in various petroleum based fuels, (iv) automotive industry as antifreezes, and (v) specialty products for use in household goods.
Typically, glycol ethers are labeled either “e-series” or “p-series” glycol ethers, depending on whether they are made from ethylene or propylene, respectively. Typically, e-series glycol ethers are found in pharmaceuticals, sunscreens, cosmetics, inks, dyes and water based paints, while p-series glycol ethers are used in degreasers, cleaners, aerosol paints and adhesives. E-series glycol ethers are higher in molecular weights, and can be used as intermediates that undergo further chemical reactions. P-series glycol ethers are generally high performance industrial solvents.
The preparation of glycol ethers has conventionally involved the generation of an alkylene oxide. For instance, one can react ethylene oxide (EO) or propylene oxide (PO) with alcohols in the e-series and p-series respectively. The glycol ether molecules can contain one or more EO or PO molecule in them. Typical alcohols used include methanol, ethanol, propanol, butanols, pentanols and hexanols. This reaction can produce glycol ethers of varying chain length depending on the molar ratio of reactions and temperature and pressures used in the reaction. Milder conditions and lower molar ratios of the alkylene oxide to alcohol will produce the monoalkylene glycyl ethers, while using more alkylene oxide and higher temperatures and pressures produce the di- and tri-alkylene glycol ethers. The products are purified by distillation. Glycol ethers can then be further reacted (esterified) with acetic acid to produce the corresponding acetate ester products. Hence, a whole family of products with multiple possible combinations exists. (See generally, e.g., Henry Chinn et al., “Marketing Research Report: Glycol Ethers,” CHEMICAL ECONOMICS HANDBOOK, 663.5000A-633.5005Q (November 2010), SRI Consulting.)
Alternatively, the alkylene oxide can be synthesized by hydration of the alkylene with hypochlorous acid followed by base catalyzed epoxidation or by direct epoxidation of the alkylene with t-butyl hydroperoxide.
In another process, glycol ethers can be produced by the reaction of an alcohol with an olefin oxide in the presence of an acidic or basic catalyst. For instance, U.S. Pat. No. 6,124,506, describes an another process of glycol ether synthesis which involves reacting an olefin oxide with an alcohol over a catalyst comprising a layered double hydroxide (LDH) clay with its layered structure intact and having interlamellar anions, at least some of which are metal anions or (poly)oxometallate anions. In a similar fashion, U.S. Pat. No. 8,748,635 B2, describes a method for the preparation of anhydrosugar ethers by alkylation of anyhydrosugar alcohols using a solid phase zeolite catalyst.
Alkylene glycols can be generated by diverse processes. For instance, in one pathway, one subjects glucose to hydrogenation and hydrogenolysis to generate propylene glycol (PG) or ethylene glycol (EG). In another pathway, one ferments glucose to produce ethanol and CO2. Ethanol is then converted to ethylene oxide with a silver catalyst, which then reacts with CO2 to form cyclic ethylene carbonate, which generates a corresponding dialkyl carbonate when reacted with an alcohol. In the dehydration/reduction step to make epoxides one requires an additional reaction step. These processes all involve multiple steps that both add to the complexity and costs of producing the desired product.
Commercial manufacturers desire a simpler, single step etherification process. The currently available processes for synthesis, however, do not enabled one to make ethers directly from alkylene glycols (e.g., ethylene glycol (EG) and propylene glycol (PG)) derived from bio-based resources. Several preceding or intervening steps must first take place. At present, no process is known that can selectively make bio-based alkylene glycols into respective mono-ethers directly without oxidization. Hence, a new process that provides a route for direct etherification of not only alkylene glycols but also cyclic (furanic) diols as starting materials would be a welcomed advance.